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Dehydration Pathways of 1-Propanol on HZSM-5 in the Presence and Absence of Water

Author:
Zhi, Yuchun, Shi, Hui, Mu, Linyu, Liu, Yue, Mei, Donghai, Camaioni, Donald M., Lercher, Johannes A.
Source:
Journal of the American Chemical Society 2015 v.137 no.50 pp. 15781-15794
ISSN:
1520-5126
Subject:
1-propanol, adsorption, aluminum, entropy, gases, propylene, silicon, zeolites
Abstract:
The Brønsted acid-catalyzed gas-phase dehydration of 1-propanol (0.075–4 kPa) was studied on zeolite H-MFI (Si/Al = 26, containing minimal amounts of extra framework Al moieties) in the absence and presence of co-fed water (0–2.5 kPa) at 413–443 K. It is shown that propene can be formed from monomeric and dimeric adsorbed 1-propanol. The stronger adsorption of 1-propanol relative to water indicates that the reduced dehydration rates in the presence of water are not a consequence of the competitive adsorption between 1-propanol and water. Instead, the deleterious effect is related to the different extents of stabilization of adsorbed intermediates and the relevant elimination/substitution transition states by water. Water stabilizes the adsorbed 1-propanol monomer significantly more than the elimination transition state, leading to a higher activation barrier and a greater entropy gain for the rate-limiting step, which eventually leads to propene. In a similar manner, an excess of 1-propanol stabilizes the adsorbed state of 1-propanol more than the elimination transition state. In comparison with the monomer-mediated pathway, adsorbed dimer and the relevant transition states for propene and ether formation are similarly, while less effectively, stabilized by intrazeolite water molecules.
Agid:
5416855